Automatic frequency control



.Fy 20,. 1948. R. J. SHANK AUTOMATIC FREQUENCY CONTROL Filed March 3,1945 nvvavrop J. SHAWN A TTOR/VEV Patented July 20, 1948 AUTOMATICFREQUENCY CONTRUL Robert J. Shank, New York, N. Y assignor to BellTelephone Laboratories, Incorporated, New York, N. ill, a corporation ofNew York Application March 3, 1945, SerialNo. 580,814

Claims.

This invention relates to automatic frequency control systems,particularly those for use in receivers of pulse reflection systems.

In the automatic frequency control of radio receivers operating insystems in which radio energy appears in pulses provision must be madeto prevent the control from causing the frequency to be shifted duringthe intervals between pulses when no radio energy for operating. thecontrol is available. For the more usual type of system this isaccomplished by giving the control circuit a time constant suiiicientlylong so that the frequency adjustment produced during any pulse willhold over until the next pulse. Such a system has the natural handicapthat the same characteristic that causes the adjustment to hold overfrom pulses to pulse also results in a sluggishness of response of thecontrol.

Such effects are particularly marked in the case of radars or otherpulse reflection-ranging and direction-indicating systems, particularlythose operating at very short waves. With such systems. one of thepreferred methods of operation is to tune the receiver in response tothe outgoing pulse so that it will have a maximum response toreflections of that pulse, which will be of the same frequency. In suchsystems the interval between transmitted pulses is comparatively longsince it must provide a time sufficient for the reflected pulses toreturn from targets at the maximum range. On the other hand thetransmitter frequency may be comparatively unstable and particularly inscanning systems where the antenna is given a physical movement thevariation in electrical conditions from position to position may be suchas to materially affect the frequency. As a result there may beconsiderable variation in frequency from pulse to pulse. Accordingly,while the requirements for the reception of reflected pulses require thefrequency regulation to be maintained over the long interval betweentransmitted pulses, other conditions require that a rapid readjustmentbe made within the short period of a transmitted pulse.

An object of the invention is to increase the speed of response ofautomatic frequency control for pulse systems.

A further object of theinvention is to provide for the receiver of apulse reflection radio system,

an automatic frequency control that responds to sudden changes infrequency of the transmitted pulses to adjust the receiver for maximumresponse to pulses of that frequency which is the characteristic of thereflected pulses and to maind tain the adjustment during the periodbetween transmitted pulses.

In accordance with a preferred embodiment of the invention in a pulsereflection system a fast acting circuit is employed for producing avoltage proportional to any deviation between the receiver inputfrequency and. the frequency to which the receiver is tuned. The outputof this circuit is supplied through an electronic switch to a storage orlong time-constant circuit which is utilized to regulate the tuning ofthe receiver. The electronic switch is closed only during periods oftransmission so that the tuning will be adjusted to the frequency of theoutgoing pulses and, due to the long time constant of the storagecircuit, will hold that adjustment for the periods between transmittedpulses. Thus, the receiver tuning is automatically adjusted by eachoutgoing pulse to give maximum response to the returning echo of thatpulse and is held in adjustment until the nextoutgoing pulse arrives.

The invention may be better understood by reference to the followingdetailed description in connection with the drawing which is a schematiccircuit diagram of a radar system embodying the invention.

In this radar system recurrent pulses of ultrahigh frequency radio Wavesare transmitted from a directional antenna it. During the periodsbetween transmitted pulses the reflections from objects under scrutinyare received in the same antenna. By the observation of the detectedpulses, for example on the screen of a cathoderay oscilloscope(notshown), the distance to the reflecting object may be determined andwhen coordinated with the orientation of the antenna, the directionthereof can also be ascertained as is well understood in the art.

The ultra-high frequency radio waves are generated by a transmitter itwhich is coupled to the antenna ill by a coaxial transmission line I2 orwave guide. The operation of the transmltter ii. is controlled by amodulator it that produces direct current pulses of the required lengthand recurrence rate. These direct current pulses are impressed on thetransmitter ll andcause the production of ultra-high frequency radiowaves during each pulse.

A duplexing unit is provided for coupling a radio receiver to thecoaxial line 92. This duplexing unit comprises a resonant cavity M tunedto the frequency of the radio oscillations produced by the transmitter Hwith a gas discharge tube mounted therein. The gas discharge tubecomprises two electrodes it connected to the walls of the cavity iii andenclosed in a partiallyevacuated vessel iii. The cavity ii is coupled tothe coaxial line it by means of an aperture i8 in the common wall of theouter conductor of the line it! and the cavity. A second aperture iii inthe opposite wall of the cavity it provides coupling to the radioreceiver.

During transmission of pulses of radio waves by the transmitter ii avoltage sufliciently high to break down the gap between the electrodesi6 is developed in the resonant cavity it. As a resuit, the path fromthe coaxial line i2 to the radio receiver has a very low impedance shuntand consequently a high attenuation is introduced into the input path tothe receiver. The attenuated pulse passes to the receiver and is used asdescribed later to operate the automatic frequency control. In theabsence of transmitted pulses the voltage is sufficiently low that thegap is not broken down and any waves received in the antenna iii aretransmitted to the receiver with practically no attenuation. Theoperation and detailed construction of such a duplexing unit isdescribed in fuller detail in the patent applications of H. T. Friis,Serial No. 474,164, filed January 30, 1943, and A. L. Samuel, Serial No.474,122, filed January 30, 1943.

The radio receiver comprises a converter 20 the intermediate frequencyoutput of which is coupled through a coaxial line 29 to a preliminaryintermediate frequency amplifier 30. The output of the intermediatefrequency amplifier 30 is branched between an additional intermediatefrequency amplifier iii and an automatic frequency control circuit it. Adetector and indicator circuit 32 is connected to the output of theintermediate frequency amplifier 3 I.

The converter Ell employs a tuned coaxial input line 2i which is coupledat one end to the resonant cavity it through the aperture i9. At theother end of the line iii there is provided a crystal detector ormodulator 22 connected to the outer conductor 01 line 2i through a radiofrequency by-pass capacitor 23 and to the inner conductor by means ofthe usual contact point. Heterodyning oscillations generated by thebeating oscillator 26 are introduced by means of the capacitor couplingplate lid. The intermediate frequency output of the converter is takenoff across the capacitor it by means of the coaxial line 29. Thisintermediate frequency output is fed to a first intermediate frequencyamplifier 30 the output of which is divided between two branches.

One of these branches includes a second intermediate frequency amplifier3i the output of which is supplied to the detector and indicator circuit32.

The other branch leads to the automatic frequency control circuit ii].This circuit includes a frequency discriminator of known type includingthe diodes iii and 42 with their respective capacitor-resistor outputcircuits t3 and 44. As is well understood in the art when the input tosuch a discriminator is of the proper frequency (the intermediatefrequency carrier) the voltages developed across the networks 43 and Mare equal and opposite producing a net voltage of zero across the outputterminals formed by connecting the two networks in series. As the inputfrequency departs from this value. the voltage of one network goes upwhile the other goes down. This produces a voltage across the outputterminals varying in sign and magnitude with the departure of the inputfrequency from the requlrcd value.

In the circuit of this invention the networks it and M are given suchshort time constants that their voltages are readily readjusted duringtitle time or a pulse at which the system is opera ed.

The output voltage comprising the sum of the voltages across thenetworks 48 and 44 is supplied to the control grid of vacuum tube 45that is operated as a cathode follower amplifier stage. The output ofthis amplifier is connected through an electron switch comprising thetriode tubes 46 and M to a storage capacitor 48. The voltage developedacross the capacitor 48 is amplified by a tube 49 and supplied throughthe connection 50 to the beating oscillator 24 where it is utilized forregulating the frequency of its output. This may be, for example, by thecontrol of the voltage of the repeller electrode of avelocity-modulation oscillator of the reflection type.

Under normal conditions both the tubes 46 and it! are blocked by virtueof having their grids maintained at or beyond cut-ofl and the voltage ofthe capacitor 48 is maintained at substantially the voltage to which itwas last adjusted. During the transmission of a pulse by the radiotransmitter Ii, a voltage obtained from the voltage-divider 5i connectedacross the output of the modulator i3 is applied to the grids of thetubes 46 and 41. This causes both tubes to be conductive and permits thevoltage of the capacitor to readjust itself to the voltage of the outputof discriminator as amplified by the amplifier 45. Upon the terminationof the transmitted pulse, the tubes 46 and 41 again becomenon-conducting and the charge on the capacitor 48 remains constant untilit is readjusted when the succeeding pulse again renders the tubes 46and 41 conducting.

What is claimed is:

1. In a pulse reflection system, a receiver for reflected pulses, afrequency discriminator, a storage capacitor, a normally open electronicswitching circuit between the output of said discriminator and saidstorage capacitor, means for closing said electronic switching circuitduring the transmission of pulses, and means responsive to the charge onsaid capacitor for regulating the tuning of said receiver.

2. In a pulse reflection system, a source of recurrent pulses to betransmitted, a receiver for reflected pulses, a frequency discriminatorhaving such a short time constant that its output follows variations infrequency between adjacent transmitted pulses, a storage capacitor.electronic switching circuit between the output of said discriminatorand said storage capacitor, means for closing said electronic switchingcircuit to interconnect said discriminator and said capacitor onlyduring the transmission of pulses, and means for regulating the tuningof said receiver in accordance with the charge on said storagecapacitor.

3. In a pulse reflection system a source of recurrent pulses ofoscillations to be radiated, a receiver for reflected pulses includingvoltage responsive means for regulating the tuning thereof, afast-acting frequency discriminator responsive to oscillations from saidsource for producing a voltage proportional to variations between thetuning of said receiver and the frequency of said oscillations, astorage capacitor, electronic switching means for supplying the outputof said frequency discriminator to said storage capacitor only duringthe transmission of pulses by said source, and means for supplying thevoltage on said storage capacitor to said voltage responsive means.

4. In a pulse reflection system a source of recurrent pulses ofoscillations to be radiated, a receiving system responsive totransmitted and reflected pulses and including voltage-responsive meansfor controlling the tuning thereof, a fastactlng frequency discriminatorfor producing a voltage proportional to variations between the tuning ofsaid receiver and the frequency of the input thereto, a storagecapacitor, an electronic switching circuit connecting the output of saidfrequency discriminator to said storage capacitor and comprising a pairof oppositely-directed space discharge paths connected in parallel,means for normally maintalmng said spacedischarge paths non-conducting,and means for rendering said space discharge paths conductive during thetransmission of pulses by said system.

5. In a pulse reflection system a source of recurrent pulses ofoscillations to be transmitted, an antenna for transmitting said pulsesand receiving reflections thereof from objects under scrutiny, areceiver comprising a beating oscillator, means for combining incomingoscillations with the output of said beating oscillator and highlyselective means responsive to the output of the combining means, aduplexing system for connecting said source and said receiverto saidantenna to permit the transmiss'ien of pulses received in said antennato said receiver with a low attenuation while highly attenuating theoutput of said source impressed on said receiver therethrough, a rapidacting frequency discriminator responsive to the output of saidcombining means for producing a voltage proportional to the deviation ofsaid output from the frequency of said highly selective means, a storagecapacitor, electronic switching means for supplying the output of saidfrequency discriminator to said storage capacitor only during thetransmission of pulses by said source, and means for regulating thefrequency of said beating oscillator in accordance with the voltage ofsaid storage capacitor.

J. Sm.

